In a typical radio communications network, wireless devices, also known as mobile stations, terminals, and/or User Equipments, UEs, communicate via a Radio Access Network, RAN, with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a radio base station, RBS, or network node, which in some networks may also be called, for example, a “NodeB”, “eNodeB” or “eNB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One radio base station may have one or more cells. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
A Universal Mobile Telecommunications System, UMTS, is a third generation mobile communication system, which evolved from the second generation, 2G, Global System for Mobile Communications, GSM. The UMTS terrestrial radio access network, UTRAN, is essentially a RAN using wideband code division multiple access, WCDMA, and/or High Speed Packet Access, HSPA, for user equipments. In a forum known as the Third Generation Partnership Project, 3GPP, telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller, RNC, or a base station controller, BSC, which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System, EPS, have been completed within the 3GPP Generation Partnership Project, 3GPP, and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network, E-UTRAN, also known as the Long Term Evolution, LTE, radio access, and the Evolved Packet Core, EPC, also known as System Architecture Evolution, SAE, core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network, RAN, of an EPS has an essentially “flat” architecture comprising radio base station nodes without reporting to RNCs.
In a radio communications network, there is a need for a radio base station to measure the channel conditions in order to know what transmission parameters to use when transmitting to a wireless device. These parameters may comprise, e.g., modulation type, coding rate, transmission rank, and frequency allocation. This also applies to uplink as well as downlink transmissions.
A scheduler that makes the decisions on the transmission parameters is typically located in the radio base station. Hence, the scheduler may measure channel properties of the uplink directly using known reference signals that the wireless devices transmit. These measurements may then form a basis for the uplink scheduling decisions that the radio base station makes, which are then sent to the wireless devices via a downlink control channel.
However, for the downlink, the radio base station is unable to measure any channel parameters in a Frequency-Division Duplex, FDD, mode of operation. In a Time-Division Duplex, TDD, mode of operation, an uplink measurement might be used in downlink. Due to calibration issues, however, these uplink measurements might not reflect the downlink channel used, and therefore, may not be well suited to be used as measurements of the downlink channel conditions.
Therefore, the radio base station must instead rely on information about the channel conditions that the wireless devices may gather and subsequently send back to the radio base station. This so-called Channel-State Information, CSI, is obtained in the wireless devices by measuring on known reference symbols, such as, Channel-State Information Reference Symbols, CSI-RS, transmitted in the downlink (see e.g. 3GPP TS 36.213 V11.4.0).
The CSI-RS resources are specifically configured for each wireless device by using Radio Resource Control, RRC, signalling. A resource is a group of resource elements in a certain subframe that occurs periodically, for instance every 20th subframe. There is a possibility to configure both Non-Zero Power, NZP, CSI-RS resources and Zero Power, ZP, CSI-RS resources. A ZP CSI-RS resource is simply an unused radio resource that can be matched to a NZP CSI-RS in an adjacent radio base station. This may then be used to improve the SINR for the CSI-RS measurements in the cell of the adjacent radio base station. However, the ZP CSI-RS resources may also be referred to or used as CSI-Interference Management, IM, resources. These are defined on the same physical locations in the time/frequency grid as the CSI-RS, but with zero power. These are intended to give a wireless device the possibility to measure the power of interfering signals without having it overlaid on top of a CSI-RS signal, which is usually much stronger than any surrounding interference.
Each wireless device may be configured with one, three or four different CSI processes. Each CSI process is associated with CSI-RS resources and CSI-IM resources. These CSI resources may be configured in the wireless device by RRC signalling that may occur periodically, see e.g. 3GPP TS 36.213 V11.4.0, 2013-09, Sections 7.2.5-7.2.6. For example, an RRC configuration message may be transmitted periodically every 5 ms, i.e. every 5th subframe. Alternatively, the RRC configuration message may be sent in an aperiodic manner, or may be triggered in a control message from the radio base station to a wireless device.
If only one CSI process is used, then it is common for the network to let the CSI-IM reflect the interference from all other radio base stations, i.e. the cell of the serving radio base station uses a ZP CSI-RS that overlaps with the CSI-IM, but in other adjacent radio base stations, there is no ZP CSI-RS on these resources. In this way, the wireless device may measure the interference from adjacent cells using measurements in the CSI-IM resource.
If more than one CSI processes are configured for the wireless device, then it is possible for the network to also configure a ZP CSI-RS in the adjacent radio base station that overlaps with a CSI-IM for the CSI process configured for the wireless device. In this way, the wireless device may feedback accurate CSI estimates also for the case when this adjacent cell is not transmitting. Hence, measurements to support coordinated scheduling between radio base stations is enabled with the use of multiple CSI processes. One CSI process feeds back CSI estimates for the full interference case and the other CSI process feeds back CSI estimates for the case when an adjacent cell, preferably a strong interfering cell, is muted. As mentioned above, up to four CSI processes may be configured for a wireless device, thereby enabling feedback of four different transmission hypotheses.
If a CSI process is configured for a wireless device, the wireless device may use an associated buffer or memory comprising one or multiple CSI measurements used to determine CSI estimates of the CSI process. However, how these CSI estimates are determined from the CSI measurements are up to the implementation of the wireless device.
In LTE, the format of the CSI reports is specified in detail and comprises CSI estimates in the form of Channel-Quality Indicator(s) (CQI), Rank Indicator (RI), and Precoding Matrix Indicator (PMI). The quality and reliability of the CSI estimates, e.g. CQI, RI and PMI, are crucial for the radio base station in order to make the best possible scheduling decisions for the upcoming downlink transmissions.